This invention relates to a process and apparatus for making thin-walled elastomeric articles.
Conventional processes for making thin-walled rubber articles, such as gloves, involve dipping a shaped former into a rubber latex, or other suitable dipping medium, to coat the former with the latex, or other medium, and curing or drying the rubber latex on the former to produce a thin-walled article, then stripping the finished article from the former. This so-called dipping process has become highly developed so that it is now possible reliably to produce very thin-walled articles such as condoms and surgeons"" gloves of extremely high quality. There have been proposals to make thin-walled elastomeric articles by other methods, such as by spraying the rubber latex (or the like) on to the former (instead of dipping) but it has not proved possible to produce such high quality products reliably in these ways and these processes are rarely used.
It is known to form a protective coating on a surface by electrostatic deposition of a coating composition thereon, and this process has been extremely successful, for example, in painting irregularly shaped objects. In U.S. Pat. No. 3,698,847, the electrostatic deposition process is unusually used to form a generally non-elastomeric hollow article by electrostatically depositing a synthetic thermoplastic resin on a collapsible former, and then withdrawing the collapsed former through a hole in the wall of the self-supporting hollow article so formed. The process is described for use for making Christmas tree ornaments.
Certain elastomeric thin-walled articles such as (but not exclusively) condoms and surgeons"" gloves, have to be of extremely high quality in order to be acceptable. For example, they must be essentially pore-free so as to provide reliable protection to the user. They must also have an adequate strength so as not to tear or otherwise disintegrate in use. They must also be thin-walled, and often of a wall thickness of less than 200 xcexcm, or even below 100 xcexcm. As stated above, the dipping process has been considerably refined over a long period of use to overcome or minimise various problems, so that such products can now be reliably and consistently produced economically and in an automated fashion, in this way.
We have now found, however, that it is possible to make thin-walled elastomeric articles reliably and without certain of the disadvantages of the well-established dipping process, by another process. In particular, we have found that such articles can be made by a process involving electrostatic deposition.
According to the present invention, there is provided a process for forming a thin-walled article, which process comprises electrostatically spraying charged particles of an elastomeric composition into a chamber containing a rigid shaped former having an earthed conductive surface whereby the charged particles of the composition are attracted to the conductive surface of the former to form a coating of the composition thereon; consolidating the coating to produce the thin-walled article on the former; and stripping the article from the shaped former.
The invention also includes apparatus for forming a thin-walled article, comprising a chamber containing a rigid shaped former having an earthed conductive surface, means for electrostatically spraying charged particles of an elastomer composition within the chamber wherein the particles can be attracted to the conductive surface and can form a coating thereon, and stripping means for stripping the thin-walled article from the shaped former after the coating has been consolidated on the former.
The provision of a satisfactory alternative to the dipping process is a surprising achievement, as is the possibility of making highly satisfactory elastomeric condoms and gloves, for example, by this technique having regard to the severity of the requirements for the production of commercially acceptable products.
In the process of the invention, the consolidation step is important. The degree of consolidation will be sufficient to ensure that the thin walled article can be stripped from the former as an integral article, without damaging the structure of the article. Further details are described hereinafter.
The elastomeric composition may be any composition that is capable of being electrostatically charged when sprayed in particulate form; it may be a liquid or it may be a solid. When the sprayed particles are in the form of a liquid, then the consolidation will generally involve some kind of solidification; when the sprayed particles are in the form of a solid, then the consolidation will generally involve some kind of amalgamation of the particles into an integral mass. The particle coating may be consolidated by a variety of different techniques. For example, the composition that is sprayed into the chamber may be curable, so that the coating can be consolidated by curing. However, it is preferred that the coating can be consolidated by evaporation: for example, the composition that is sprayed may include a solvent that can be evaporated to leave a solid coating on the former.
When the composition is a solid, the coating will normally comprise a plurality of solid particles electrostatically adhered to the surface of the former. These particles may be amalgamated to form the thin-walled article by, for example, heating them to cause them to soften or melt and coalesce, and then cooling the coalesced coating.
The consolidation may occur naturally, without any specific treatment. However, it is usually desirable or essential to speed up the consolidation process with some form of treatment. We prefer to carry out the consolidation using heat treatment. Although the heat treatment may be carried out in an oven, it is especially preferred that the heat treatment is carried out by inducing electrical currents in the former.
When the composition is a liquid, it can be atomised to form a plurality of small liquid particles, which can be electrostatically charged. The liquid form of the composition may include an organic or aqueous solvent, or it may comprise solid particles dispersed or dissolved in a liquid. Suitable liquid compositions comprise film-forming elastomeric materials, and include: polyurethanes (solvent and water based solutions and emulsions); fluoroelastomers; triblock copolymer rubbers (SBS, SIS, SEBS), styrene-butadiene rubber latex (SBR); acrylonitrile-butadiene rubber latex (NBR); carboxylated-SBR, carboxylated-NBR, and other carboxylated rubber latices; styrene-vinylpyridine-butadiene terpolymer latex; natural rubber latex (NR); polychloroprene rubber latex (CR); and butyl rubber latex (isobutene-isoprene rubber).
The preferred elastomeric compositions are those containing thermoplastic elastomeric polyurethanes, more preferably aqueous dispersions of thermoplastic elastomeric polyurethanes, and most preferably aqueous dispersions of thermoplastic elastomeric polyurethanes containing a cosolvent such as methyl isobutyl ketone or the like.
When the composition is a solid, it is almost invariably provided in the form of solid powder.
The particles of the composition can be sprayed using a conventional electrostatic spray gun. This can impart an electrostatic charge to the particles of the composition. When the composition is a liquid, the spray gun may be adapted to atomize the liquid to produce a plurality of small particles. When the composition is a solid, the spray gun is usually provided with the solid particles in powder form. Typically, the particle size of the sprayed composition is dependent upon the parameters of the spraying apparatus which, in turn, will be dependent upon the composition being sprayed.
The former may be rotated or otherwise moved while it is being coated, in order to help provide a uniform coating.
In a preferred aspect of the invention, the elastomeric composition, in the form of a solution or dispersion in water or organic solvent, is electrostatically sprayed by means of a rotary atomiser. This has the advantage of producing very fine particles which, when consolidated on the rigid former, are capable of producing a high quality thin-walled film with good surface finish and freedom from holes. In a particularly preferred aspect, the elastomeric composition is dispersed in water containing a small quantity of a solvent for the composition. Rotary atomisers are known in the art and no further description thereof will therefore be given herein.
In some circumstances it is desirable to respray the former with more particles of the composition, or with particles of another composition, after the solidification step, but before the stripping step. This may be necessary in order to produce an article of a particular thickness. It is possible to change the composition that is sprayed during some, or all, of the spraying and consolidation steps; this makes possible the production of articles having varying properties and colours. The second (and any further) spraying can be made with an elastomeric or non-elastomeric composition, as desired.
The size of the electrostatic charge imparted to the particles of the composition, the solids content of the composition, and the position and speed of passage (if any) of the former in the spray chamber are just some of the parameters in electrostatic spraying that can be varied in order to alter the thickness of the thin-walled article formed on the former.
The preferred method of consolidating the coating is by a heat treatment. Thus, it is preferred that the apparatus according to the invention includes a heating means. In one embodiment, the heating means comprises an oven, which is used at a temperature sufficient to consolidate the coating on the former. In this embodiment, the former can be retained in the oven for a time sufficient to consolidate the coating.
In another embodiment, the consolidating means may comprise a device for inducing an electrical current in the former, whereby the former can be heated to a temperature at which the coating can be consolidated. This embodiment has the advantage that it is not necessary to provide a separate oven for the consolidation.
The former is substantially rigid and may be solid or hollow. It is possible for the former to comprise a non-conductive material having a conductive coating. However, we prefer that substantially all the former is of a conductive material. Most preferably, the former comprises a conductive metal, such as stainless steel or aluminium. Alternatively glass may be used, with reduced efficiency, in the presence of moisture.
Preferably, the apparatus includes a conveyor to carry the former into and through the spray chamber, and from the spray chamber to the stripping means. If an oven or other consolidation means is provided, then the conveyor may carry the former to the oven or other consolidation means, before carrying it to the stripping means. It is desirable to provide a plurality of separate formers on the conveyor.
It is an important feature of the method and apparatus according to the invention that the thin-walled article can be stripped from the shaped former after consolidation of the coating. For this reason, the composition and the former need to be made of materials such that the coating does not adhere to the former after consolidation. This is different from conventional electrostatic spraying processes, where the sprayed coating is intended to adhere to the article being sprayed. However, it is important to bear in mind that when the article is formed using multiple spraying and consolidation steps, only the innermost layer of the article needs to be formed of a material that can be stripped from the former.
The present invention can be used to provide high quality thin-walled articles of selected thickness. The thickness is typically in the range 20 to 200, preferably 40 to 100, microns. Using the invention, it is possible to make thin-walled articles with a uniform thickness, even when the shape of the article is quite complex.
The present invention has the advantage that there is no dripping of the material coated on the former, due to the attraction between the material and the former. Furthermore, any necessary additives may be added at the spray head, so there is no reduced potlife.
It is often desirable to change the composition, for example to alter the colour or fragrance. The invention makes possible a quick and clean changeover from one composition to another, with a minimal amount of wastage.
Moreover, defects caused by particulates in the dipping tank are eliminated with electrostatic spraying, due to the bulk material not being exposed to external objects, eg formers, and being stored in covered containers.
The invention can be used for the production of a wide variety of thin-walled articles including primarily condoms and gloves, but also other contraceptive devices, medical device application aids (eg colonoscopy coverings), medical device covers, balloons and elastomeric bags.